Equine Protozoal Myeloencephalitis
Dr. Philip Johnson
To prevent EPM, every effort should be made to lessen the chances that opossums might interact closely with horses. Certainly, opossums (and birds) should never be tolerated inside barns, in places where horse feed is stored and especially at locations where horses are fed. In addition, minimize vermin and insects (cockroaches), which may potentially act to spread the infective sporocysts. Special electrical fencing can be used to inhibit the movement of opossums onto the farm premises – low fencing requires that grass/vegetation will be kept very low adjacent to the fence (high maintenance). Opossums can be kept out of pastures by placing a double-stranded electric fence around the outside of the pasture fence. The fence-posts should be glass fiber or plastic, and the wires should be strung at five and ten inches off the ground. This is a high maintenance fence because all vegetation has to be regularly trimmed away from the fence so as not to ground it.
Horses that are allowed to graze adjacent to or in wooded areas (opossum habitat) are at higher risk. Therefore, whenever possible, this type of land should not be used for horse accommodation.
Opossums might be trapped and removed to distant locations. It is easy to catch opossums in live traps. Two to four traps should be placed around the barn, and the traps can be baited with just about anything. Canned cat food, especially the seafood type, probably works the best.
After trapping opossums, they may be destroyed, or they may be driven miles away and released. Opossums live in a home range of 10 to 50 acres, and they spend their entire lives within that range. Females bear two or three litters per year, usually with two young per litter. Thus, per female, there will be four to six new opossums per year. Please note that trapping and killing opossums might be subject to local law enforcement regulations, so check with local wildlife authorities prior to undertaking a catching and moving operation. The local Department of Conservation will likely be very helpful in this regard.
In addition, the following can also be done to help:
Birds are difficult control but should be kept out of barns (cats help to discourage them).
Big dogs will discourage opossums from entering horse premises.
An important point in controlling opossums is eliminating food sources. All grain should be stored in tightly closed containers. Dog and cat food should not be left outside. If dogs or cats must be fed in the barn, feed only the amount the animals can eat at one time.
Other possible food sources for opossums include fruit trees, fallen fruit, spoiled meat and garbage. Dead animals should be disposed of expeditiously and not allowed to rot. Food material should NOT be available to opossums in areas where horses are kept. In order to reduce the number of opossums that might be attracted to a horse farm or barn, all uneaten and discarded animal feed should be picked up and cleaned away, spilled grain should not be allowed to accumulate, animal cadavers should be disposed of in an expeditious manner, and fallen fruit should be picked up quickly. Grain, sweet feed, and pelleted feed intended for horses should be thoroughly inspected for possible contamination by opossum feces and stored in air-tight containers (grain bins).
Heat-treated commercial horse rations (such as steam-flaked, pelleted, or extruded feeds) represent a safe source of feed because the infective sporocysts are destroyed by the high temperature. It has not been determined whether cold-processed commercial horse feed is free of sporocysts.
The horse owner should develop vigilance for opossum feces in the horse barn, commonly found along the top of wooden struts and beams, which should signify a need to increase surveillance for opossums, improve the general level of hygiene in the barn, and to reconsider the extent to which preventive measures have been undertaken.
If possible, horses should not be allowed to drink from slow-running streams or stagnant ponds, especially in wooded areas. Fresh water should be provided in clean containers or through automatic waterers. Watering sources for horses in paddocks or at pasture should be inspected periodically to ensure that fecal contamination by wild animals and birds is not occurring.
Other small wild mammals (rats and mice), certain birds, and insects (such as cockroaches) might also play a role in the mechanical dispersion of infective sporocysts from opossum feces into the horse's environment; for this reason, preventive measures should also include strategies to minimize the numbers of rats and mice in barns and in places where horse feed is stored. Wild birds should not be tolerated in horse barns and mesh-net screens may be used to impede bird access into barns. Bird feeders should be eliminated. Regular treatments of the equine environment by an insect exterminator should be considered as part of any EPM prevention strategy. Allowing cats to live in the horse barn might deter wild birds.
It is fortunate that, although a majority of horses are exposed to Sarcocystis neurona in certain parts of the United States and Canada (leading to a positive blood test for EPM), only a small number of exposed horses actually develop neurological signs. At this time, highly effective preventive measures (such as vaccination) have not been designed and the strategies suggested here are simply logical approaches to management which are largely based on what little we know about the natural history of this parasite and the opossum.
Interestingly, EPM does not appear to affect either donkeys or mules. Out of concern for the problem, horse owners are electing to replace their horses with donkeys and mules.
A vaccine manufacturer has recently announced that a vaccine targeted against Sarcocystis neurona will be available shortly. It should be noted that, at this time, the company has not yet demonstrated that this vaccine is able to protect horses against EPM.